The present invention relates to a color image reading apparatus and, more particularly, to a color image reading apparatus suitable for a color scanner, a color facsimile machine, or the like capable of reading color image information on an original with high precision by utilizing a focusing optical system, a color separating means, and three line sensors formed on a single substrate.
Various conventional apparatuses have been proposed wherein color image information on an original is focused on a line sensor such as a CCD, and color image information is digitally read by utilizing an output signal from the line sensor.
FIG. 1 is a schematic view showing an example of a conventional color image reading apparatus. Referring to FIG. 1, when a light beam from a color image on an original surface 1 is to be focused on a line sensor surface (to be described later) by a focusing lens 20, the beam is color-separated into red (R), green (G), and blue (B) components through a 3P prism 21. The color-separated components are guided onto line sensors 22, 23, and 24 comprising CCDs, respectively. Color images formed on the surfaces of the line sensors 22, 23, and 24 are scanned line by line, thereby reading the color image in units of colors.
FIG. 2 is a view showing the main part of a color image reading apparatus proposed in Japanese Laid-Open Patent Application No. 62-234106.
Referring to FIG. 2, when a beam from a color image on an original surface 1 is to be focused on a line sensor surface (to be described later) by a focusing lens 25, the beam is separated into three beams corresponding to three colors through two color separation beam splitters 26 and 27 each added with a dichroic selective light-transmitting film. A color image based on three color light components is focused on line sensor surfaces of a so-called monolithic 3-line sensor 28 having three line sensors 28a, 28b, and 28c formed on a single substrate surface, as shown in FIG. 3. The color image is scanned line by line to read the image in units of color components. Distances S1 and S2 (FIG. 3) between the line sensors fall within the range of about 0.1 to 0.2 mm, and a size defined by W1.times.W2 of each element is given as 7 .mu.m.times.7 .mu.m or 10 .mu.m.times.10 .mu.m.
The color image reading apparatus shown in FIG. 1 requires three independent line sensors, and high precision is required. In addition, the 3P prism which is difficult to manufacture is also required, resulting in a complicated apparatus at high cost. The focusing beam must be aligned with each of the line sensors, and three separate alignment operations must be performed, resulting in cumbersome assembly.
In the color image reading apparatus shown in FIG. 2, if the thickness of each of the beam splitters 26 and 27 is defined as X, the distance between the line sensors becomes 2.sqroot.2x. If a preferable distance between the line sensors falls within the range of about 0.1 to 0.2 mm, the thickness X of each of the beam splitters 26 and 27 becomes about 35 to 70 .mu.m.
It is generally difficult to manufacture a beam splitter having excellent flatness and such a small thickness. When a beam splitter having such a small thickness is used, .the optical characteristics of a color image formed on the line sensor surfaces are undesirably degraded.
The distances between the central line sensor 28b and one end line sensor 28a and between the central line sensor 28b and the other end line sensor 28c are generally equal to each other in opposite directions or an integer multiple of a pixel size (W2 in FIG. 3) in a subscanning direction due to the following reason.
As is apparent from FIG. 4, when image reading is performed by the monolithic 3-line sensor through only the focusing lens 25, positions of the original simultaneously read by the three line sensors 28a, 28b, and 28c are three different positions 51a, 51b, and 51c, as shown in FIG. 4. For this reason, the three color components (i.e., R, G, and B components) at a given position cannot be simultaneously read and must be matched with each other upon reading of these three color components.
In order to achieve this, each of the distances S1 and S2 between the line sensors is set to be an integer multiple of each pixel size W2, and a corresponding redundant line memory is arranged. For example, the G and R signals are delayed with respect to the B signals to relatively easily obtain a color composite signal. Therefore, the distance between the line sensors is set an integer multiple, as described above. However, if the above redundant line memory is used to correspond to each interline distance, a plurality of expensive line memories must be used, resulting in inconvenience.
It is an object of the present invention to provide a color image reading apparatus wherein a volume hologram is used as a color-separating means, and a focusing optical system and the color-separating means are appropriately set to simplify the apparatus as a whole, so that a color image can be digitally read with high precision in units of color components, e.g., R, G, and B color components.
A color image reading apparatus according to the present invention is characterized in that when a color image is to be focused by a focusing optical system on a surface of a reading means including three line sensors formed on a single substrate surface to cause the reading means to read the color image, a color-separating means comprising a volume hologram for color-separating an incident beam into three color light components by a diffraction action in a direction perpendicular to an arrangement direction of three line sensors is arranged in an optical path between an exit pupil of the focusing optical system and the surface of 10 the reading means, thereby causing the reading means to read the color image based on the components color-separated by the color-separating means.
In particular, according to the features of the present invention, the volume hologram color-separates the incident beam into three color components, i.e., red, green, and blue light components, and principal wavelengths of the green and blue light components satisfy a Bragg diffraction condition.